The present invention relates to a fixing apparatus employing an induction heating method (an electromagnetic inducing heating method) to press, heat and fix toner images, which are obtained by developing electrostatic latent images with thermally meltable toner.
Conventionally, for image forming apparatuses, which are used for high speed, high image quality copiers and printers, frequently employed is a method of developing electrostatic latent images, typified by an electrophotographic method.
In this method, electrostatic images are developed with thermally meltable toner, and after the obtained toner images are transferred onto a recording medium, the toner images are fixed with heat and pressure. As a heat source used in a fixing apparatus, a halogen lamp has been commonly used.
However, in recent years, an apparatus applying an electromagnetic induction system, which is characterized by direct heating of a heat receiving member (a heat roller, etc.) and high heating efficiency, has been extensively studied. (For example, as described in Tokkaihei 11-190950.) However, it has become more and more apparent that an electromagnetic induction fixing apparatus has some problems and without countermeasures, the apparatus is not likely to come into practical use.
Conventionally, as heat-receiving members, an electro conductive roller, a magnetic metal roller, or a roller comprised of multi layers of Ni and Cu, etc. on insulating body have been used. However, there are many cases where ferromagnetic materials have low electric conductivity, or materials of high electric conductivity have low relative magnetic permeability, and large eddy current cannot be obtained, and heating efficiency is not so high as expected.
Further, in cases of using a halogen lamp, temperature of the heat-receiving member is measured and power supply is cut off in accordance with the measured value in order to control the temperature. In this case, if the power supply is cut off after the temperature reaches a predetermined value, a large overshoot is often caused by a delay of the action. Also, in electromagnetic induction fixing, wherein material with exceedingly higher Curie temperature, than usual fixing temperature, is used for the heat receiving member and core, a large overshoot as aforementioned cases arises in cases where, a heat receiving material with low thermal capacity is used and high electrical power is supplied, aiming for a rapid temperature rise.
Alternatively, in cases where correct temperature cannot be detected because of failure in a temperature sensor, a thermal runaway occurs and neighboring parts would be damaged, and in the worst case, would be burned. For these cases, decreasing the overshoot as far as possible, and in the case of overheating, restricting a heat generating rate to low levels are important subjects.
Further, depending on the size of the recording medium (or recording paper), required heat distribution is different, and this causes an abnormal increase of temperature at areas distant from the center of the heat-receiving member, therefore, equalization of this temperature distribution is another subject.
Furthermore, in an apparatus wherein a conventional way of winding an induction coil is applied, uneven temperature distribution is caused in the circumferential direction of the heat-receiving member, therefore in order to equalize the temperature, increasing the previous revolutions of the heat-receiving member (fixing roller, etc.) is required. Still more, when a thick heating roller with large heat capacity is used, a long warming up time is needed, and when a thin heating roller or a roller with low heat conductivity is used, unevenness of temperature tends to be caused.
The present invention is performed to overcome the above-described shortcomings.
Namely, the first object of the present invention is to provide an inducing heating type fixing apparatus with short warming up time (WUT) by using high magnetic permeability and high electrical conductivity alloy which generates eddy current effectively.
The second object of the present invention is to provide an inducing heating type fixing apparatus, wherein temperature overshoot is minimized, and in case of overheating the heat generation rate is suppressed.
The third object of the present invention is to provide an inducing heating type fixing apparatus in which temperature unevenness in the circumferential direction of a heat-receiving member (fixing roll, etc.) is decreased.
Incidentally, the fixing apparatus utilizing electromagnetic induction usually comprises a heating means and a pressure means, and both means contact each other to grip and transport the toner image bearing recording medium and fix the image. Said heating means comprises roll or belt-shaped rotatable heat receiving member in which provided a helical coil or a vertically wound coil, and a core if necessary. Alternating current, whose frequency is preferably 5 kHz or higher, is supplied to said coil to generate an alternating magnetic field, and eddy current is induced in said heat receiving member, then, the eddy current loss causes a heat generation in the heat receiving member. With this heat, cooperating with pressure, which is applied elastically by said pressure means, toner images are fixed onto the recording medium.
The above fixing apparatus utilizing the electromagnetic induction has superior characteristics to a fixing apparatus, which uses an infrared heater or a nichrome wire heater, in that; the apparatus can be made in small size and light weight, and almost free from a fear of fire, the power consumption loss is small, a warm up time is very short after starting a supply of high frequency alternating current, etc. Therefore, studies and developments of the fixing apparatus utilizing the electromagnetic induction have been performed recently. For example, in Tokkaisyo 54-39645(official report 1), and Tokkaihei 9-106207(official report 2), etc., various technologies concerning the fixing apparatus utilizing the electromagnetic induction are proposed.
Said official report 1 relates to a heating means for a fixing apparatus wherein high frequency AC current is supplied to a helically wound coil, and eddy current is induced in a heat-receiving member, which is made of electro conductive ferromagnetic material and rotatably provided around the coil, and generated heat is used for fixing. And the official report 1 describes that, said coil in the heating means is wound coarsely at the area near the center in the longitudinal direction of the heating means, and wound thickly at the area near the side ends, and the heat-receiving member is evenly heated in the longitudinal direction.
Further, the official report 2 relates to a fixing apparatus wherein high frequency AC power is supplied to a plurality of coils wound vertically around divided cores, and eddy current is induced in a heat-receiving member, which is made of electro conductive ferromagnetic material and rotatably provided around the coil, and the generated heat is used for fixing. And the report 2 describes that said coils provided near the center in the longitudinal direction of the heat receiving member are connected in parallel, and coils provided near the ends are connected in series to said coils connected in parallel, and by this configuration, the temperature distribution of the heat-receiving member in the longitudinal direction is equalized.
However, conventional fixing apparatuses utilizing an electromagnetic induction described in the official report 1 and 2, etc. are aiming for an equalization of the temperature distribution in a certain fixed operation mode, and can not deal with the cases of various modes in the process of forming fixed images where various power consumption distributions are required, such as a warming up mode, a large sized paper transporting mode, a small sized paper transporting mode, and a standby mode, etc. Especially, there have been problems such that, in a large sized paper transporting mode, temperature at side parts of paper in the perpendicular direction to the paper transport direction becomes too low and causes fixing failure, or in a small sized paper transporting mode, temperature of the heat receiving member in no-paper transport area becomes too high and unnecessary electrical power is consumed.
The present invention is proposed on the above background, and the fourth object of the present invention is to provide a fixing apparatus that does not cause a fixing failure nor unnecessary power consumption, and always produces a properly fixed images and achieves a power saving, even when various modes required in the process of forming fixed images are changed, especially when transported paper size is changed.
Inventers of the present invention found that the objects of the present invention described above can be achieved by employing the constructions hereinafter described.
(1) An inducing heating type fixing apparatus, which fixes, with pressure and heat, a toner image obtained by developing an electrostatic latent image with thermally meltable toner, wherein an electro conductive hollow rotating member is used, and inside of which is provided a sendust layer.
(2) An inducing heating type fixing apparatus, which fixes, with pressure and heat, a toner image obtained by developing an electrostatic latent image with thermally meltable toner, wherein an electro conductive hollow or solid rotating member is used, and on the outer surface of which is provided a sendust layer.
(3) An inducing heating type fixing apparatus, which fixes, with pressure and heat, a toner image obtained by developing an electrostatic latent image with thermally meltable toner, wherein a hollow or solid column rotating member is used and the material of which is sendust.
(4) An inducing heating type fixing apparatus, which fixes, with pressure and heat, a toner image obtained by developing an electrostatic latent image with thermally meltable toner, wherein a heat-receiving member has a heat generating layer which is comprised of at least electro conductive magnetic material, and the Curie temperature of the material of this heat generating layer is higher than the predetermined fixing temperature, and is lower than the heat resistance temperature of the fixing apparatus.
(5) An inducing heating type fixing apparatus, which fixes, with pressure and heat, a toner image obtained by developing an electrostatic latent image with thermally meltable toner, wherein magnetic material, used as the coil core, is divided into separate plural parts in the direction perpendicular to the recording paper transport direction, and said ferrite core parts are composed of materials of different Curie temperature.
(6) A fixing apparatus as described in paragraph (5), wherein the Curie temperature of said separated magnetic core, which is provided at an area where heating is required when small size paper is transported, is different from that of the core in other areas where heating is not required during that period, and a core of higher Curie temperature is used at the heat requiring area than that of other cores.
(7) The fixing apparatus as described in paragraph (5), wherein the Curie temperature of said core is higher than the predetermined fixing temperature and lower than the heat resistance temperature of the fixing apparatus.
(8) An inducing heating type fixing apparatus, which fixes, with pressure and heat, a toner image obtained by developing an electrostatic latent image with thermally meltable toner, wherein an induction coil (exciting coil), which is provided in a heat-receiving member, is wound around a high magnetic permeability magnetic core so as to form a radial configuration in cross section.
(9) An inducing heating type fixing apparatus, which fixes, with pressure and heat, a toner image obtained by developing an electrostatic latent image with thermally meltable toner, wherein at least two vertically wound induction coils, which are provided in a heat receiving member and are wound parallel in the longitudinal direction, and are aligned at different radial directions of said heat receiving member.
The first object of the present invention is achieved by the constructions (1), (2) and (3), the second object is achieved by (4), (5), (6) and (7), and the third object is achieved by (8) and (9).
The fourth object can be achieved by the following constructions.
(10) A fixing apparatus comprising a hollow cylindrical heat-receiving member made of electro conductive material, and a heating means including an alternating current power source and a coil, wherein said coil is separated to a coil which heats the area near the center of the heat receiving member in the longitudinal direction, and coils which heat both side areas of the heat receiving member, and at least one of these coils is provided outside the heat-receiving member.
(11) A fixing apparatus as described in (10), wherein said coils, which heat both side areas, are connected in series.
(12) A fixing apparatus as described in (10) or (11), wherein said coil, which heats an area near the center of the heat receiving member in the longitudinal direction, and said coils, which heat both side areas, are supplied power from separate power sources and controlled independently.
(13) In a fixing apparatus comprising; a hollow cylindrical heat-receiving member made of electro conductive material; and a heating means including an alternating current power source, a core and a vertically wound coil; wherein said core is movably provided.
(14) A fixing apparatus as described in (13), wherein said core is divided into plural cores in the longitudinal direction, and at least one of said cores is installed movably in the longitudinal direction.